1. Field of the Invention
The present invention relates to a plasma treatment apparatus, and in particular to a plasma treatment apparatus that carries out plasma treatment on semiconductor wafers, which are articles to be treated.
2. Description of the Related Art
These days, in semiconductor device manufacturing processes, plasma treatment such as etching, sputtering, and CVD (chemical vapor deposition) is carried out on semiconductor wafers, which are articles to be treated.
Various types of plasma treatment apparatus are used for carrying out such plasma treatment, but capacitively coupled parallel plate plasma treatment apparatuses as described below are particularly commonly used.
As shown in FIG. 6, such a capacitively coupled parallel plate plasma treatment apparatus is comprised of a cylindrical vacuum vessel having therein an internal chamber (hereinafter referred to merely as the “chamber”) 61 in which plasma treatment is carried out on a semiconductor wafer 60, and a pair of parallel plate electrodes, specifically an upper electrode 62 that is disposed in an upper part of the chamber 61 and a lower electrode 63 that is disposed in a lower part of the chamber 61.
The capacitively coupled parallel plate plasma treatment apparatus carries out plasma treatment on the semiconductor wafer 60, which is placed on the lower electrode 63 so as to face the upper electrode 62. The plasma for carrying out the plasma treatment is produced from a treatment gas introduced into the chamber 61 using a high-frequency electric field formed between the parallel plate electrodes.
In the case of etching a film such as an oxide film on the semiconductor wafer 60 using the capacitively coupled parallel plate plasma treatment apparatus, the inside of the chamber 61 is put into a medium pressure state, and a plasma of medium density is produced, whereby optimum radical control is made possible; as a result, a suitable plasma state can be obtained, and hence etching with a high degree of stability and reproducibility can be realized.
In this case, the plasma produced on the semiconductor wafer 60 diffuses isotropically through ambipolar diffusion toward grounded parts such as the upper electrode 62, a chamber side wall 64 and an exhaust plate 65. When the diffused plasma reaches such a grounded part, a high-frequency return current is generated, and this high-frequency return current flows back from the grounded part to the lower electrode 63.
However, in the chamber 61, in general the distance from the semiconductor wafer 60 on the lower electrode 63 to the chamber side wall 64 is greater than the distance from the semiconductor wafer 60 to the upper electrode 62, and hence the degree of diffusion of the produced plasma through the isotropic diffusion is increased in the direction of the chamber side wall 64. As a result, the plasma density around a peripheral part of the semiconductor wafer 60 near to the chamber side wall 64 becomes lower than the plasma density at a central part of the semiconductor wafer 60 facing the upper electrode 62, i.e. the plasma density in the chamber 61 becomes non-uniform, and hence there is a problem that the semiconductor wafer 60 etching rate distribution also becomes non-uniform.
Moreover, in addition to the isotropic diffusion of the plasma described above, it is known that another factor in the plasma density becoming non-uniform is non-uniformity in the high-frequency electric field distribution in the radial direction of the semiconductor wafer 60 due to standing waves produced on the electrode surface of the parallel plate electrode to which high-frequency electrical power is applied due to higher harmonic waves, which are reflected waves from the plasma, and the inductance of the electrode surface.
A plasma treatment apparatus disclosed in Japanese Laid-open Patent Publication (Kokai) No. 2000-323456 is known as an apparatus in which non-uniformity in the plasma density is eliminated by eliminating such non-uniformity in the high-frequency electric field distribution.
This plasma treatment apparatus has, as an upper electrode, an electrode plate that is comprised of an outside part that is made of a conductor or a semiconductor, and a central part that is constituted from a dielectric member or a high-resistance member. High-frequency electrical power is applied to the upper electrode.
With this plasma treatment apparatus, in the case that the central part is a dielectric member, the inductance component in the radial direction of the surface of the electrode plate that contacts the plasma can be cancelled out by the capacitance component of the dielectric member, and in the case that the central part is a high-resistance member, much of the high-frequency electrical power is consumed as Joule heat in the high-resistance member, and hence there is a drop in the electric field strength at the central part of the surface of the electrode plate that contacts the plasma, whereby non-uniformity in the high-frequency electric field distribution can be eliminated, and hence the plasma density can be made more uniform.
However, in the plasma treatment apparatus disclosed in Japanese Laid-open Patent Publication (Kokai) No. 2000-323456 described above, the material differs between the outside part and the central part of the upper electrode, to which the high-frequency electrical power is applied, and hence a potential difference is prone to occurring between the outside part and the central part; abnormal discharges may thus occur, resulting in the upper electrode wearing away rapidly, and due to this problem widespread use of the apparatus has not been possible.